A team of ornithologists from the National University of Singapore, Birdtour Asia Ltd., and the University of Queensland has described a new and cryptic species of the hawk-cuckoo genus *Hierococcyx* from Borneo — a large island shared by the Malaysian states of Sabah and Sarawak, the Indonesian state of Kalimantan, and the sultanate of Brunei.
*Hierococcyx* is a small genus of birds within the cuckoo family Cuculidae. First established in 1845, the genus currently includes nine scientifically recognized species. Commonly known as hawk-cuckoos, these birds can be found across South, Southeast, and East Asia.
“Within the hawk-cuckoos of the genus *Hierococcyx*, two tropical and subtropical Asian species form a close-knit pair,” said lead author Dr. Frank Rheindt from the National University of Singapore and his colleagues. “They are variably merged into one species or recognized as two monotypic species: the large hawk-cuckoo (*Hierococcyx sparverioides*) and the Bock’s hawk-cuckoo or dark hawk-cuckoo (*Hierococcyx bocki*).”
These hawk-cuckoos are found in forests from the Himalaya and China to Southeast Asia. *Hierococcyx sparverioides* is the more widespread taxon, breeding widely across montane tropical, subtropical, and even temperate mainland Asia and wintering to the south. Meanwhile, *Hierococcyx bocki* ranges in the montane forests of Sundaland.
Historically, species delimitation in *Hierococcyx* cuckoos, as in most other birds, has been guided by plumage differences. However, there is only subtle plumage variation across these two species.
“Since the 1990s, the taxonomy of many tropical and subtropical bird species complexes has undergone a revolution with increasing insights into the importance of bioacoustic characters for species delimitation,” the researchers noted. “Species limits within numerous tropical and subtropical Asian bird species complexes have been re-drawn on the basis of bioacoustic evidence.”
In their new study, Dr. Rheindt and his co-authors aimed to investigate the taxonomy of the *Hierococcyx sparverioides*–*Hierococcyx bocki* pair and uncover evidence for cryptic species. To do this, they used an integrative approach based on bioacoustic, plumage, and morphometric data.
The team analyzed a total of 107 sound recordings of these two species obtained from online sound libraries. They also assessed plumage differences and morphometric traits measured from a series of museum specimens.
Their analysis unearthed a distinct, previously unnamed population of *Hierococcyx bocki* residing on Borneo that can be separated from other populations chiefly by its unique three-syllabled main song.
Given the importance of vocalizations in cuckoo taxonomy, the researchers described this population as a new species to science: *Hierococcyx tiganada*.
“The new species from Borneo closely resembles its sister species *Hierococcyx bocki* from Sumatra and Peninsular Malaysia,” the researchers explained. “The two are almost identical visually, but adults of the new species have a slightly grayer mantle, especially the upper mantle towards the crown, leading to a distinct lack of contrast between a gray crown and more brownish back that is apparent in *Hierococcyx bocki*.”
“The best distinguishing feature between the two species is the structure of their brainfever song: it is always three-syllabled in the new species and two-syllabled in *Hierococcyx bocki*.”
*Hierococcyx sparverioides* differs from the new species in the same characters that set it apart from *Hierococcyx bocki*. It also has a two-syllabled brainfever song, a larger body size, is less dark-gray in front of the eye, has a browner mantle less contrasting with the crown, and exhibits distinct dark-rufous streaking on a whitish background on the upper breast (compared to solid rufous with no streaking in the new species).
According to the scientists, *Hierococcyx tiganada* inhabits montane rainforests above 1,000 meters elevation.
“Despite the alarming conservation status of many Southeast Asian birds which rely on rainforest habitat, *Hierococcyx tiganada* is probably not threatened,” they said. “While Borneo’s lowland rainforests have suffered considerable loss over the last three decades, much of the montane forest in the heart of Borneo survives thanks to its remoteness and inaccessibility to logging companies and other extractive industries.”
*Hierococcyx tiganada* is relatively well known from three general locations in Sabah and one in Sarawak. There are extremely few records from Kalimantan, and the species appears to be genuinely absent from the Meratus Mountains in South Kalimantan.
However, the species is probably ubiquitous over vast areas of montane northern Borneo, often near the border between Kalimantan, Sarawak, and Sabah. The lack of widespread records likely reflects poor observer coverage away from well-established birdwatching areas.
*Hierococcyx tiganada* is the first *Hierococcyx* species to be described in the 20th/21st centuries.
“This discovery highlights the continuing importance of bioacoustic research in biodiversity discovery and further elevates the biogeographic importance of Borneo,” the authors concluded.
Their paper will be published in the *Journal of Asian Ornithology*.
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**Reference:**
Frank E. Rheindt et al. 2025. A new and cryptic species of hawk-cuckoo (genus *Hierococcyx*) from Borneo. *Journal of Asian Ornithology* 41: 34.
https://www.sci.news/biology/hierococcyx-tiganada-14289.html